Shift workers, pilots and long distance east-west airline passengers all suffer from a common problem, a functional disorder resulting from the desynchronization of activity and alertness cycles with the sleep-wake cycle which is ordinarily controlled by the light-dark cycle. When the body expects it to be dark, sunlight is experienced instead. As alertness and sleepiness are controlled by the light-dark cycle, and as the internal body clock is susceptible to being reset by light during subjective night, it is desirable to avoid light capable of resetting the biological clock (the SCN) to times that are inappropriate for activities in the local environment. Thus, if one wants to maintain an alertness cycle which is out of phase with the light-dark cycle, or if one wants to adjust rapidly to time zone changes, it is necessary to avoid light capable of resetting this internal clock at times when the light will influence the resetting of the clock in an undesirable manner.
This desynchronization of the sleep-wake cycle from the desired activity cycle results in sleepiness at times when alertness is necessary, abnormal sleep patterns, inability to concentrate, reduced mental capacity (both in judgement and in memory), and agitation and/or depression in some individuals. These symptoms, experienced by travellers rapidly crossing a number of time zones, sometimes referred to jet-lag, can cause danger to persons in an aircraft, if the person suffering the jet-lag is the pilot, or reduced functioning by businessmen or government negotiators upon arrival.
If this desynchronization is maintained over long periods of time, as is experienced by rotating shift workers or night workers, it is sometimes referred to as shift lag. This condition is associated with an increased incidence of health problems such as gastrointestinal disorders including ulcers, and cardiovascular disorders such as heart disease, in addition to the reduced functioning referred to above. This reduced functioning is associated with increased accidents and reduced performance resulting in estimated economic costs of seventy billion dollars per year.
In order to avoid the jet-lag or shift lag, sufferers (in laboratory trials) have been told to avoid all sunlight and/or wear welders' goggles which effectively reduces the amount of light to a level at which a person cannot function, drive a car, read instruments, read, fly an airplane, etc.